Cathode active material for lithium secondary battery

ABSTRACT

Disclosed is a lithium secondary battery, which is low in capacity loss after overdischarge, having excellent capacity restorability after overdischarge and shows an effect of preventing a battery from swelling at a high temperature.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of application Ser. No. 13/618,048,filed Sep. 14, 2012, which is a Continuation of application Ser. No.12/558,100 filed on Sep. 11, 2009, which was a continuation-in-part ofU.S. application Ser. No. 10/950,104, filed Sep. 24, 2004, now U.S. Pat.No. 7,695,867, issued Apr. 13, 2010, which was a continuation-in-part ofU.S. application Ser. No. 10/478,802, filed Nov. 25, 2003, now U.S. Pat.No. 7,282,300, issued Oct. 16, 2007, which was the National Stage ofInternational Application No. PCT/KR02/02267, filed Dec. 2, 2002, andwhich claims priority to Korean Application Nos. 10-2003-0066865 and10-2003-0066866, both filed Sep. 26, 2003, Korean Application No.10-2002-15713, filed Mar. 22, 2002, and Korean Application No.10-2002-36438, filed Jun. 27, 2002, and all the benefits accruingtherefrom under 35 U.S.C. 119, the content of which is incorporatedherein by reference in its entirety.

TECHNICAL FIELD

The present invention relates to a lithium secondary battery, which islow in capacity loss after overdischarge, having excellent capacityrestorability after overdischarge and shows an effect of preventing abattery from swelling at a high temperature.

BACKGROUND ART

Recently, as mobile communication industries and information electronicindustries progress in various technologies, a light-weight,high-capacity lithium secondary battery is increasingly in demand.However, a lithium secondary battery may ignite and explode due toextreme heat emission when it is over-charged or is in a short circuitstate. Moreover, when a lithium secondary battery is overdischargedbelow a normal voltage range, its capacity is significantly reduced,preventing forthcoming use.

For these reasons, a safety device like a protection circuit, a PTCelement, etc., has been attached to a lithium secondary battery sincelithium secondary batteries were first developed. However, suchprotection circuits, PTCs, etc., are not preferable because they areexpensive and take up a large volume, thereby increasing the price,volume and weight of a battery. Therefore, batteries with a reducedmanufacturing cost and an increased battery capacity without using sucha protection circuit, PTC, etc., are very much in demand.

Conventionally, an organic or an inorganic additive is used in anon-aqueous electrolyte, or the outer structure of a battery is changedfor the purpose of ensuring battery safety when a battery isover-charged or has short-circuited. However, when a battery isoverdischarged below an adequate voltage, even if one tries to chargethe battery again, the battery capacity is so significantly reduced thatthe battery are no longer capable of charge/discharge.

Conventional lithium secondary batteries developed hitherto have astructure in which discharge is limited and terminated by an anodeduring overdischarge. Particularly, when a non-aqueous lithium secondarybattery is first charged, a solid electrolyte interface (SEI) film isformed on the surface of an anode. In this case, a great amount oflithium ions released from a cathode are used and thus the amount of Liparticipating in charge/discharge is reduced. When over-dischargingoccurs in the state in which the amount of Li is reduced, activated Lisites in the cathode are not fully occupied and the cathode voltage isnot decreased below a certain voltage. Therefore, discharge isterminated by the anode (see FIG. 1).

Meanwhile, a battery capacity is significantly reduced by the followingreasons. A battery voltage is defined by a difference between a cathodevoltage and an anode voltage. Additionally, a battery is continuouslydischarged at a low electric current, even after the battery voltage isdecreased below a general-use voltage. At this case, due to theconsumption of Li ion in the anode, the cathode voltage is no longerreduced and thus it is slowly decreased. On the other hand, the anodevoltage rapidly increases and eventually rises to 3.6 V, at which pointa copper foil used as an anode collector is oxidized. Thus, the copperfoil is dissolved in a copper ion state to contaminate an electrolyte.After that, when the battery is re-charged the copper ion is attachedagain to the surface of the anode and thus the anode active materialbecomes unusable. Therefore, if oxidization of the copper foil occurs,the battery capacity is rapidly reduced after overdischarge, so that thebattery becomes unusable.

Accordingly, it is desirable to develop a battery, discharge of which islimited by a cathode, so that the battery capacity may not besignificantly reduced after overdischarge. Further, a new method formaking such a cathode-limited battery is very much in demand.

DISCLOSURE OF THE INVENTION

As mentioned above, there is a problem that the voltage of an anodehaving a relatively high irreversible capacity increases rapidly, whenoverdischarging occurs, and thus copper ions are dissolved from an anodecollector, so that charge/discharge cycles may not progresssuccessfully. In order to prevent the increase of the voltage in ananode during overdischarge, it is desirable to increase the irreversiblecapacity of a cathode so as to decrease the voltage of the cathode morerapidly. For the purpose of increasing the irreversible capacity of acathode, the present invention adopted a method of adding an additivehaving a high irreversible capacity to a cathode.

We have found that, when a lithium nickel oxide represented by thefollowing formula 1 is used as an additive for a cathode activematerial, a phase transition occurs in the lithium nickel oxide tocontrol irreversible reactions in a cathode and an anode, and thus thebattery capacity is not significantly reduced after overdischarge.

Therefore, the present invention has been made based on this finding. Itis an object of the present invention to provide a battery, thedischarge of which is limited by a cathode, using a cathode activematerial comprising a lithium nickel oxide represented by the followingformula 1 as an additive, so that the battery capacity may not besignificantly reduced after overdischarge.

Meanwhile, the lithium nickel oxide may cause swelling of a battery at ahigh temperature depending on its added amount. With regard to this, wehave found that when a lithium nickel oxide represented by the followingformula 1, in which nickel is partially substituted with other elements,is used as an additive for a cathode active material, the batterycapacity is not significantly reduced after overdischarge whilemaintaining overall performance of the battery, and furthermore, it ispossible to obtain excellent capacity restorability after overdischargeand to prevent a battery from swelling at a high temperature. We havealso found that when a lithium nickel oxide represented by the following1 that is coated with an oxide other than lithium nickel oxides, is usedas an additive for a cathode active material, the battery capacity isnot significantly reduced after overdischarge while maintaining overallperformance of the battery, and furthermore, it is possible to obtainexcellent capacity restorability after overdischarge and to prevent thebattery from swelling at a high temperature, in this case too.

According to an aspect of the present invention, there is provided acathode active material for a lithium secondary battery containing alithium transition metal oxide capable of lithium ionintercalation/deintercalation, which further comprises a lithium nickeloxide represented by the following formula 1 in which nickel ispartially substituted with other elements (with the proviso that y isnot 0), or a lithium nickel oxide represented by the following formula 1that is surface-coated with an oxide other than lithium nickel oxides,as an additive in an amount of 0.1 to 9 parts by weight based on 100parts by weight of the total cathode active material:Li_(2+x)Ni_(1−y)M_(y)O_(2+a)  [formula 1]wherein, x is a number satisfying −0.5≦x≦0.5, y is a number satisfying0≦y<1, a is a number satisfying 0≦a<0.3, and M is at least one elementselected from the group consisting of P, B, C, Al, Sc, Sr, Ti, V, Zr,Mn, Fe, Co, Cu, Zn, Cr, Mg, Nb, Mo and Cd.

According to still another aspect of the present invention, there isprovided is a lithium secondary battery comprising the above-describedcathode active material.

The lithium secondary battery according to the present inventioncomprises: (a) a cathode comprising the cathode active materialaccording to the present invention, (b) an anode, (c) a separator, and(d) a non-aqueous electrolyte containing a lithium salt and anelectrolyte compound.

The present invention will be explained in detail hereinafter.

The lithium nickel oxide used as an additive for a cathode activematerial according to the present invention is represented by thefollowing formula 1:Li_(2+x)Ni_(1−y)M_(y)O_(2+a)  [formula 1]wherein, x is a number satisfying −0.5≦x≦0.5, y is a number satisfying0≦y<1, a is a number satisfying 0=a<0.3, and M is at least one elementselected from the group consisting of P, B, C, Al, Sc, Sr, Ti, V, Zr,Mn, Fe, Co, Cu, Zn, Cr, Mg, Nb, Mo and Cd.

Preferably, the oxide other than lithium nickel oxides, used forsurface-coating of the lithium nickel oxide represented by formula 1 isan oxide or composite oxide of at least one element selected from thegroup consisting of Al, Mg, Si, P, C, Sc, Ti, V, Cr, Mn, Fe, Co, Cu, Zn,Mo, Zr and Nb. Particular examples of the oxide or composite oxideinclude Al₂O₃, ZrO₂, AlPO₄, SiO₂, TiO₂ and MgO but are not limitedthereto.

The compound represented by formula 1 preferably belongs to the spacegroup Immm. More preferably, in the stereostructure of the compound, aNi/M composite oxide forms a tetra-coordinated planar structure (Ni,M)O₄and two tetra-coordinated planar structures facing to each other shareone side (formed by O—O), thereby forming a primary chain as a whole.Additionally, the compound represented by formula 1 preferably has thefollowing lattice constants: a=3.7±0.5 Å, b=2.8±0.5 Å and c=9.2±0.5 Å,wherein α=90°, β=90° and γ=90°.

As shown in FIGS. 6 to 9, a lithium nickel oxide of formula 1 in whichnickel is partially substituted with other elements shows an X-raydiffraction pattern (FIGS. 6 to 8) similar to that of Li₂NiO₂ (FIG. 9).This indicates that although nickel in the lithium nickel oxide ispartially substituted with other elements, the lithium nickel oxide isnot changed in structure.

In the structure of the compound represented by formula 1, Li ionintercalation/deinterlation occurs during the first charge/dischargecycle, wherein the oxidation number of Ni or M is changed from +2 to +4and the structure of Li_(2+x)Ni_(1−y)M_(y)O_(2+a) experiences a phasetransition into Li_(2+x−z)Ni_(1−y)M_(y)O₂ (wherein 0≦z<2).

For example, LiNiO₂ has a lattice structure that belongs to the spacegroup R3-m (trigonal hexagonal), wherein a=b, i.e., a is the same as b,c is different from them, alpha=beta=90° and gamma=120°.

The compound represented by formula 1 deintercalates at least one moleof lithium ion during the first charge cycle, however, on and afterdischarge of the first cycle, it becomes a substance capable of lithiumion intercalation/deintercalation in an amount of one mole or less.

For example, in the case of Li₂NiO₂, contrary to LiNiO₂, one mole ormore of lithium ions are donated to an anode during charge and one moleor less of lithium ions are accepted by a cathode during discharge.Therefore, the discharge efficiency (the first discharge capacity/thefirst charge capacity×100) of Li₂NiO₂ in the first charge/dischargecycle is about 40% or less. In the case of the compound represented byformula 1, Li_(2+x)Ni_(1−y)M_(y)O_(2+a), discharge efficiency in thefirst charge/discharge slightly varies with the content of the metal Msubstituting for Ni.

Accordingly, when the lithium nickel oxide represented by formula 1 isused in a cathode as an additive for a cathode active material, thecathode active material composition according to the present inventionshows a large difference between initial charge capacity and initialdischarge capacity. This irreversible capacity provides lithium ions atleast in such an amount as to compensate for an irreversiblelithium-consuming reaction in an anode caused by the SEI film formationon the surface of an anode during the first charge. Therefore, it ispossible to compensate for the high irreversible capacity of the anodeat the first charge/discharge cycle.

In addition, the cathode active material composition according to thepresent invention, which comprises a lithium transition metal oxidecapable of lithium ion intercalation/deintercalation and the lithiumnickel oxide represented by formula 1 can inhibit the capacity reductioncaused by overdischarge, by virtue of the irreversibility of the lithiumnickel oxide represented by formula 1 during the first charge/dischargecycle. This mechanism is shown in FIG. 1.

A battery voltage is defined by the difference of electric potentialsbetween a cathode and an anode. Overdischarge of a battery continuouslyproceeds until the battery voltage becomes 0 V, at which point theelectric potentials of a cathode and an anode are the same.

As mentioned above, in general, the voltage of an anode having arelatively high irreversible capacity increases rapidly, whenoverdischarging occurs, and thus copper ions are dissolved from an anodecollector, so that charge/discharge cycles may not progresssuccessfully. The above-described overdischarging problem results fromthat an irreversibility of lithium transition metal oxide used as acathode active material in general is smaller than that of carbon-basedanode active material. In order to prevent the increase of the voltagein an anode during overdischarge, it is desirable to increase theirreversible capacity of a cathode so as to decrease the voltage of thecathode rapidly. For the purpose of increasing the irreversible capacityof a cathode, the present invention adopted a method of adding anadditive having a high irreversible capacity to a cathode.

According to the present invention, a cathode for a lithium secondarybattery is formed by adding to a cathode active material containing afirst lithium transition metal oxide capable of lithium ionintercalation/deintercalation, a second lithium transition metal oxideof which an irreversible capacity, (1−discharge capacity/chargecapacity) in the first charge/discharge cycle is greater than that ofthe first lithium transition metal oxide, as an additive. As a result,it is possible to regulate terminal voltage of a cathode when theelectric potential difference (voltage) between a cathode and an anodeis 0V during overdischarge.

The irreversible capacity of the additive in the first charge/dischargecycle should be greater than 4%, i.e., the irreversible capacity of thegeneral cathode active material. The preferable irreversible capacity ofthe additive is 30% or greater.

Additionally, in order to reduce the amount of the additive, it ispreferable that the irreversible capacity (actual capacity per se ratherthan capacity ratio) of the additive is relatively high.

Preferably, the used amount of the additive can provide irreversiblylithium ions at least in such an amount as to compensate for anirreversible lithium-consuming reaction in an anode caused by the SEIfilm formation on the surface of an anode during the first charge. Sincethe irreversible capacity of the anode is 8% in general, the preferableirreversible capacity of the additive is 8% or more in order to reducethe amount of the additive.

According to the present invention, when the compound represented byformula 1 is added to a cathode to the extent of compensating for theirreversible capacity of an anode, it is possible to obtain veryexcellent performance in an overdischarge test of a SCF (safety circuitfree) battery, which does not need a protection circuit. The SCF batteryhas peaked the interest to battery production companies recently.

Meanwhile, when the lithium nickel oxide is added to the cathode of alithium secondary battery as an additive for the cathode activematerial, Ni in the lithium nickel oxide, which is present in anoxidized state with a valence of +4, may react with an electrolyteduring charge to generate gas. Therefore, swelling of a battery mayoccur at a high temperature depending on the added amount of the lithiumnickel oxide. However, the anode and cathode should be in close contactto each other because a non-aqueous lithium secondary battery has lowion conductivity. Accordingly, when swelling of a battery occurs, thecontact degree between an anode and a cathode may decrease, therebyincreasing electric resistance.

Further, the problem of swelling of a battery at a high temperature inthe case of Li₂NiO₂ is severe, contrary to LiNiO₂.

To solve this problem, according to the present invention, there isprovided a cathode active material for a lithium secondary batterycontaining a lithium transition metal oxide capable of lithium ionintercalation/deintercalation, which further comprises a lithium nickeloxide represented by formula 1 in which nickel is partially substitutedwith other elements (with the proviso that y is not 0), or a lithiumnickel oxide represented by formula 1 that is surface-coated with anoxide other than lithium nickel oxides, as an additive.

In the compound represented by formula 1 in which nickel is partiallysubstituted with at least one element M selected from the groupconsisting of P, B, C, Al, Sc, Sr, Ti, V, Zr, Mn, Fe, Co, Cu, Zn, Cr,Mg, Nb, Mo and Cd, bonds originally present in the lithium nickel oxideare substituted with stronger bonds so that a battery can be preventedfrom swelling at a high temperature.

The compound represented by formula 1 may be prepared by reacting asalt, metal salt, organo-metallic salt or oxide of at least one elementselected from the group consisting of P, B, C, Al, Sc, Sr, Ti, V, Zr,Mn, Fe, Co, Cu, Zn, Cr, Mg, Nb, Mo and Cd together with a lithium saltand nickel salt by using a reaction method such as a solid phasereaction, co-precipitation method and a sol-gel method. Methods otherthan the above-mentioned methods may also be used to prepare thecompound represented by formula 1.

Additionally, the additive for a cathode active material according tothe present invention may be obtained by surface-coating a lithiumnickel oxide of formula 1 (including y=0) with an oxide other thanlithium nickel oxides, such as an oxide or composite oxide of at leastone element selected from the group consisting of Al, Mg, Si, P, C, Sc,Ti, V, Cr, Mn, Fe, Co, Cu, Zn, Mo, Zr and Nb. The surface-coating asdescribed above can prevent Ni in an oxidized state with a valence of +4from reacting with an electrolyte and thus prevent gas generation causedby the reaction between Ni and an electrolyte. Therefore, it is possibleto prevent swelling of a battery at high temperature.

The lithium nickel oxide represented by formula 1 that is surface-coatedwith an oxide other than lithium nickel oxides may be prepared bysurface-coating a lithium nickel oxide represented by formula 1 with asolution containing a salt, metal salt or organo-metallic salt of atleast one element selected from the group consisting of Al, Mg, Si, P,C, Sc, Ti, V, Cr, Mn, Fe, Co, Cu, Zn, Mo, Zr and Nb, mixed in the formof sol-gel or dissolved in an organic solvent or water.

Surface-coating methods may include a precipitation method, a filteringmethod, a vacuum drying method, a CVD (Chemical Vapor Deposition)method, a sputtering method, etc., but are not limited thereto. Theprecipitation method is carried out by introducing a lithium nickeloxide into a solution containing a compound containing at least oneelement selected from the group consisting of Al, Mg, Si, P, C, Sc, Ti,V, Cr, Mn, Fe, Co, Cu, Zn, Mo, Zr and Nb, for example, aluminumisopropoxide, zirconium propoxide, aluminum nitrate, magnesium acetate,etc., mixed in the form of sol-gel or dissolved in an organic solvent orwater, so that precipitate slurry can be obtained. The filtering methodis carried out by separating the slurry by using a depressurizationfilter. Additionally, the vacuum drying method is carried out bycompletely drying the solvent contained in the slurry in a vacuum drier.

The additive for a cathode active material according to the presentinvention is preferably used in an amount of 0.1 to 9 parts by weightbased on 100 parts by weight of the cathode active material. When thecontent of the additive for a cathode active material is less than 0.1parts by weight, the voltage of an anode increases in advance of thereduction of the voltage of a cathode during an overdischarge test.Therefore, when the anode voltage reaches a certain range of voltagemore than 3.6V (at which point a copper foil as an anode collector isoxidized), the problem of copper ion dissolution may occur in the casesof pouch type batteries, prismatic batteries and cylindrical batteries.As a result, a battery may be damaged so that charge/discharge cycles ofthe battery are thwarted after overdischarge. Additionally, when thecontent of the additive for a cathode active material is more than 10parts by weight, the voltage of a cathode decreases rapidly during anoverdischarge test, and thus a battery may show an excellent effect inthe overdischarge test. However, reduction of an electrolyte may occurin the surface of the cathode and the battery capacity may be decreased.Therefore, in order to solve both problems in a cathode and an anode,the cathode potential preferably ranges from 2 V to 3.6 V and the anodepotential is preferably 3.6 V or less, when the full cell voltagebecomes 0 V.

The overdischarge test is carried out as follows: discharging to 3.0V at300 mA, discharging to 2.7V at 3 mA and discharging to 0V at 1 mA.

The cathode active material used in the present invention is any one ofconventional cathode active materials, however, it is preferable to usea lithium transition metal oxide. For example, at least one lithiumtransition metal oxide selected from the group LiCoO₂, LiNiO₂,consisting of LiMnO₂, LiMn₂O₄, Li(Ni_(a)Co_(b)Mn_(c)) O₂ (wherein 0<a<1,0<b<1, 0<c<1 and a+b+c=1), LiNi_(1−d)Co_(d)O₂, LiCo_(1−d)Mn_(d)O₂,LiNi_(1−d)Mn_(d)O₂ (wherein 0≦d<1), Li(Ni_(x)Co_(y)Mn_(z))O₄ (wherein0<x<2, 0<y<2, 0<z<2 and x+y+z=2), LiMn_(2−n)Ni_(n)O₄, LiMn_(2−n)Co_(n)O₄(wherein 0<n<2) LiCoPO₄, LiFePO₄, etc., may be used and LiCoO₂ ispreferably used.

As an anode active material, graphite, carbon, lithium metal and alloys,etc., which are capable of lithium ion intercalation/deintercalation,may be used. Preferably, artificial graphite is used. The anode maycomprise a binder, the binder being preferably PVDF (Polyvinylidenefluoride) or SBR (Styrene Butadiene Rubber).

As a separator, a porous separator is preferably used. For example, apolypropylene-, a polyethylene- or a polyolefin-based porous separatormay be used, but it is not limited thereto.

The electrolyte used in the present invention is a non-aqueouselectrolyte and may comprise a cyclic carbonate and a linear carbonate.Examples of the cyclic carbonate include ethylene carbonate (EC),propylene carbonate (PC) and gamma-butyrolactone (GBL). Preferredexamples of the linear carbonate include at least one carbonate selectedfrom the group consisting of diethyl carbonate (DEC), dimethyl carbonate(DMC), ethylmethyl carbonate (EMC) and methylpropyl carbonate (MPC).

Additionally, the electrolyte used in the present invention comprises alithium salt in addition to the carbonate compound. More particularly,the lithium salt is preferably selected from the group consisting ofLiClO₄, LiCF₃SO₃, LiPF₆, LiBF₄, LiAsF₆ and LiN(CF₃SO₂)₂.

The lithium secondary battery according to the present invention ismanufactured by a conventional method, i.e., by inserting a porousseparator between a cathode and an anode and introducing an electrolyte.

Preferably, the lithium secondary battery according to the presentinvention is a cylindrical can-type battery, a prismatic battery or apouch-type battery.

THE ADVANCED EFFECT

As can be seen from the foregoing, according to the present invention,the compound represented by formula 1 is added to a cathode as anadditive for a cathode active material to improve overdischargecharacteristics. The additive for a cathode active material can providelithium ions at least in such an amount as to compensate for theirreversible capacity of an anode. Accordingly, the anode voltage can beprevented from increasing during overdischarge by increasing theirreversibility of a cathode to cause the cathode voltage to bedecreased rapidly, so that a battery capacity restorability of 90% ormore may be obtained after overdischarge.

Additionally, according to the cathode active material for a lithiumsecondary battery, which comprises a lithium nickel oxide represented byformula 1 in which nickel is partially substituted with other elements(with the proviso that y is not 0), or a lithium nickel oxiderepresented by formula 1 that is surface-coated with an oxide other thanlithium nickel oxides, as an additive for the cathode active material,capacity of a battery is not significantly reduced after overdischargewhile maintaining overall performance of the battery. Further, it ispossible to obtain excellent capacity restorability and to prevent abattery from swelling at a high temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing cathode potential and anode potential, beforeand after using the additive for a cathode active material according tothe present invention.

FIG. 2 is a graph showing the result of a three-pole test of the bi-cellobtained from Comparative Example 1.

FIG. 3 is a graph showing the result of a three-pole test of the bi-cellobtained from Example 1 according to the present invention.

FIG. 4 is a graph showing the result of a three-pole test of the bi-cellobtained from Example 2 according to the present invention.

FIG. 5 is a graph showing the result of a three-pole test of the bi-cellobtained from Example 3 according to the present invention.

FIG. 6 is a diagram showing the X-ray diffraction pattern of theadditive for a cathode active material prepared by the method asdescribed in Example 4.

FIG. 7 is a diagram showing the X-ray diffraction pattern of theadditive for a cathode active material prepared by the method asdescribed in Example 5.

FIG. 8 is a diagram showing the X-ray diffraction pattern of theadditive for a cathode active material prepared by the method asdescribed in Example 6.

FIG. 9 is a diagram showing the X-ray diffraction pattern of theadditive for a cathode active material prepared by the method asdescribed in Example 7.

FIG. 10 is a graph showing the results of charge/discharge of batteriesprepared by the methods as described in Examples 4 to 7.

FIG. 11 is a graph showing the change in thickness of the pouch typebatteries containing the additives for a cathode active materialprepared by the methods as described in Examples 4 to 7, after thebatteries are stored at a high temperature.

FIG. 12 is a graph showing the overdischarge test results of the pouchtype batteries containing the additives for a cathode active materialprepared by the methods as described in Examples 4, 5 and 7.

FIG. 13 is a graph showing the overdischarge test result of the pouchtype battery manufactured by the method as described in ComparativeExample 2.

FIGS. 14 and 15 are results according to SEM (scanning electronmicroscope) and EDS (Energy Dispersive X-ray Spectrometer) analysis ofthe additive for a cathode active material prepared by the method asdescribed in Example 8, respectively.

FIGS. 16 and 17 are results according to SEM and EDS analysis of theadditive for a cathode active material prepared by the method asdescribed in Example 11, respectively.

FIGS. 18 and 19 are results according to SEM and EDS analysis of theadditive for a cathode active material prepared by the method asdescribed in Example 12, respectively.

FIG. 20 is a graph showing the charge/discharge capacity of thebatteries obtained from Examples 8 to 12.

FIG. 21 is a graph showing the change in thickness of the pouch typebatteries containing the additives for a cathode active materialprepared by the methods as described in Examples 8 to 12, after thebatteries are stored at a high temperature.

FIG. 22 is a graph showing the overdischarge test results of the pouchtype batteries obtained from Examples 8 to 12.

FIG. 23 is a sectional view of a general pouch type battery used in thefollowing Examples and Comparative Examples, wherein reference numeral 1is a pouch, 2 is a lid, 3 is a cathode, 4 is an anode, 5 is a cathodecurrent collector, 6 is an anode current collector, 7 is a separator, 8is lithium metal, and 9 is an electrolyte.

MODE FOR CARRYING OUT THE INVENTION

Reference will now be made in detail to the preferred embodiments of thepresent invention. It is to be understood that the following examplesare illustrative only and the present invention is not limited thereto.

EXAMPLE 1

A pouch-type bi-cell was manufactured by a conventional method. LiCoO₂was used as a cathode active material and Li₂NiO₂ was added as anadditive in the amount of 2 parts by weight based on 100 parts by weightof the cathode active material. More particularly, 78.4 wt % of LiCoO₂,1.6 wt % of Li₂NiO₂, 10 wt % of KS-6 (conductive agent) and 10 wt % ofPVDF (binder) were added to NMP as a solvent to form cathode mixtureslurry, and then the slurry was coated on an Al collector to obtain acathode. Additionally, artificial graphite and copper were used as ananode active material and an anode collector, respectively, and anEC/PC/DEC-based solution containing 1M LiPF₆ was used as an electrolyteto obtain a battery by a conventional method.

EXAMPLE 2

Example 1 was repeated to obtain a battery, except that Li₂NiO₂ as anadditive for a cathode active material was used in the amount of 5 partsby weight based on 100 parts by weight of the cathode active material.

EXAMPLE 3

Example 1 was repeated to obtain a battery, except that Li₂NiO₂ as anadditive for a cathode active material was used in the amount of 9 partsby weight based on 100 parts by weight of the cathode active material.

COMPARATIVE EXAMPLE 1

Example 1 was repeated to obtain a battery, except that the additive fora cathode active material (Li₂NiO₂) was not used in the cathode.

EXPERIMENTAL EXAMPLE 1

A three-pole test is performed for each of the bi-cells according toExamples 1 to 3 and Comparative Example 1. The results are shown inFIGS. 2 to 5. Generally, when the performance of a cell is evaluated bycapacity, concept of full cell voltage is used. The full cell voltage isdefined as the difference between the voltage of a cathode and that ofan anode in the case of a cell having two electrodes of one cathode andone anode. A three-pole cell system includes lithium metal inserted intoa cell as a reference electrode, in addition to a cathode and an anode.Such a three-pole system is used to determine the behavior of thecathode and anode in an actual cell during charge/discharge cycles,based on the reference electrode (lithium metal), by measuring thevoltage difference between the reference electrode (lithium metal) andthe cathode and the voltage difference between the reference electrode(lithium metal) and the anode, respectively.

As shown in FIG. 2, in an overdischarge test, Comparative Example 1shows a plateau (represented by a circle) in which copper iondissolution occurs after the voltage of an anode increases. On the otherhand, as shown in FIGS. 3 to 5, each of Examples 1 to 3 shows no plateaucorresponding to copper ion dissolution.

EXAMPLE 4

Lithium oxide as a lithium salt, nickel oxide as a nickel salt andaluminum nitrate as an aluminum salt used to substitute for nickel weremixed in an adequate equivalent ratio and then reacted in a solid phaseat 600° C. to obtain Li₂Ni_(0.97)Al_(0.03)O₂ as an additive for acathode active material. X-ray diffraction pattern of the additive for acathode active material is shown in FIG. 6.

Next, 92.12 wt % of LiCoO₂, 1.88 wt % of the additive for a cathodeactive material, 3 wt % of super-P (conductive agent) and 3 wt % of PVDF(binder) were added to NMP as a solvent to form cathode mixture slurry,and then the slurry was coated on an Al collector to obtain a cathode.Additionally, artificial graphite and copper were used as an anodeactive material and an anode collector, respectively, and anEC/PC/DEC-based solution containing 1M LiPF₆ was used as an electrolyteto obtain a battery by a conventional method. Charge/discharge capacityof the battery is shown in FIG. 10.

Further, the pouch type battery was charged to 4.2V at 0.2C, heated fromroom temperature to 90° C. for 1 hour, stored at 90° C. for 4 hours andthen cooled back to room temperature for 1 hour. At this time, change inthickness of the battery was measured. The result is shown in FIG. 11.The overdischarge test result of the battery is shown in Table 1 andFIG. 12.

EXAMPLE 5

Example 4 was repeated to obtain Li₂Ni_(0.97)Mg_(0.03)O₂ as an additivefor a cathode active material, except that magnesium was used as anelement substituting for nickel. X-ray diffraction pattern of theadditive for a cathode active material is shown in FIG. 7. A battery wasmanufactured by using the additive for a cathode active material in thesame manner as described in Example 4. Charge/discharge capacity of thebattery is shown in FIG. 10.

By using the same method as described in Example 4, change in thicknessof the pouch type battery was measured after the battery was stored at ahigh temperature. The result is shown in FIG. 11. Additionally, theoverdischarge test result of the battery is shown in Table 1 and FIG.12.

EXAMPLE 6

Example 4 was repeated to obtain Li₂Ni_(0.97)B_(0.03)O₂ as an additivefor a cathode active material, except that boron was used as an elementsubstituting for nickel. X-ray diffraction pattern of the additive for acathode active material is shown in FIG. 8. A battery was manufacturedby using the additive for a cathode active material in the same manneras described in Example 4. Charge/discharge capacity of the battery isshown in FIG. 10.

By using the same method as described in Example 4, change in thicknessof the pouch type battery was measured after the battery was stored at ahigh temperature. The result is shown in FIG. 11. Additionally, theoverdischarge test result of the battery is shown in Table 1.

EXAMPLE 7

A lithium salt and a nickel salt were mixed in an adequate equivalentratio and reacted together in an electric furnace to obtain Li₂NiO₂ asan additive for a cathode active material. X-ray diffraction pattern ofthe additive for a cathode active material is shown in FIG. 9. A batterywas manufactured by using the additive for a cathode active material inthe same manner as described in Example 4. Charge/discharge capacity ofthe battery is shown in FIG. 10.

By using the same method as described in Example 4, change in thicknessof the pouch type battery was measured after the battery was stored at ahigh temperature. The result is shown in FIG. 11. Additionally, theoverdischarge test result of the battery is shown in Table 1 and FIG.12.

COMPARATIVE EXAMPLE 2

Example 4 was repeated to obtain a battery, except that the additive fora cathode active material was not added to the cathode active material.The overdischarge test result of the battery is shown in Table 1 andFIG. 13.

TABLE 1 Discharge Discharge Capacity capacity before capacity afterrestorability after overdischarge overdischarge overdischarge (0.2 C)/mA(0.2 C)/mA (%) Ex. 7 732 682 93.2% Comp. Ex. 2 728 464 63.7% Ex. 4 742699 94.2% Ex. 5 738 687 93.1% Ex. 6 729 673 92.3%

As can be seen from Table 1 and FIGS. 10 and 12, each of the batteriesobtained from Examples 4 to 7 shows similar charge/discharge efficiency.Capacity of each battery is not significantly reduced even afteroverdischarge and each battery has excellent capacity restorabilityafter overdischarge. Additionally, as shown in Table 1 and FIG. 13, thebattery obtained from Comparative Example 2 (to which the additive for acathode active material for improving overdischarge characteristicsaccording to the present invention is not added) shows pooroverdischarge characteristics compared to other batteries.

However, as can be seen from FIG. 11, Example 4 usingLi₂Ni_(0.97)Al_(0.03)O₂ as an additive for a cathode active material forimproving overdischarge characteristics shows little change in batterythickness. Examples 5 and 6 using Li₂Ni_(0.97)Mg_(0.03)O₂ andLi₂Ni_(0.97)B_(0.03)O₂ as an additive for a cathode active material,respectively, show smaller change in battery thickness compared toExample using Li₂NiO₂ as an additive for a cathode active material. Thisindicates that each additive for a cathode active material for improvingoverdischarge characteristics according to Examples 4 to 6 providesexcellent effect of preventing a battery from swelling at hightemperature compared to the additive according to Example 7.

EXAMPLE 8

3 mol % of aluminum isopropoxide based on Li₂NiO₂ was dissolved inethanol. Li₂NiO₂ obtained by reacting a lithium salt with a nickel saltin an electric furnace at 600° C. was added thereto to form slurry. Theslurry was filtered through a depressurization filter to obtain afiltered product and the filtered product was completely dried in anoven at 80° C. to obtain a final product. The final product was analyzedby SEM and EDS. The results are shown in FIGS. 14 and 15, respectively.

Next, 90.24 wt % of LiCoO₂, 1.88 wt % of the above-described finalproduct as an additive for a cathode active material, 3 wt % of super-P(conductive agent) and 3 wt % of PVDF (binder) were added to NMP as asolvent to form cathode mixture slurry, and then the slurry was coatedon an Al collector to obtain a cathode. Additionally, artificialgraphite and copper were used as an anode active material and an anodecollector, respectively, and an EC/PC/DEC-based solution containing 1MLiPF₆ was used as an electrolyte to obtain a battery by a conventionalmethod. Charge/discharge capacity of the battery is shown in FIG. 20.

Further, the pouch type battery was charged to 4.2V at 0.2C, heated fromroom temperature to 90° C. for 1 hour, stored at 90° C. for 4 hours andthen cooled back to room temperature for 1 hour. At this time, change inthickness of the battery was measured. The result is shown in FIG. 21.The overdischarge test result of the battery is shown in Table 2 andFIG. 22.

EXAMPLE 9

Example 8 was repeated to obtain an additive for a cathode activematerial and to manufacture a battery, except that the slurry wasseparated from the solvent by means of a precipitation method instead ofusing a depressurization filter. Charge/discharge capacity of thebattery is shown in FIG. 20.

By using the same method as described in Example 8, change in thicknessof the battery was measured after the battery was stored at a hightemperature. The result is shown in FIG. 21. Additionally, theoverdischarge test result of the battery is shown in Table 2 and FIG.22.

EXAMPLE 10

Example 8 was repeated to obtain an additive for a cathode activematerial and to manufacture a battery, except that the slurry was driedin a depressurization drier instead of filtering with a depressurizationfilter. Charge/discharge capacity of the battery is shown in FIG. 20.

By using the same method as described in Example 8, change in thicknessof the battery was measured after the battery was stored at a hightemperature. The result is shown in FIG. 21. Additionally, theoverdischarge test result of the battery is shown in Table 2 and FIG.22.

EXAMPLE 11

Example 8 was repeated to obtain an additive for a cathode activematerial, except that zirconium propoxide was used instead of aluminumisopropoxide. The additive was analyzed by SEM and EDS. The results areshown in FIGS. 16 and 17, respectively. A battery was manufactured byusing the additive in the same manner as described in Example 8.Charge/discharge capacity of the battery is shown in FIG. 20.

By using the same method as described in Example 8, change in thicknessof the battery was measured after the battery was stored at a hightemperature. The result is shown in FIG. 21. Additionally, theoverdischarge test result of the battery is shown in Table 2 and FIG.22.

EXAMPLE 12

A lithium salt was reacted with a nickel salt in an electric furnace at600° C. to obtain Li₂NiO₂. Li₂NiO₂ was analyzed by SEM and EDS. Theresults are shown in FIGS. 18 and 19, respectively. A battery wasmanufactured in the same manner as described in Example 8, except thatLi₂NiO₂ was used as an additive for a cathode active material.Charge/discharge capacity of the battery is shown in FIG. 20.

By using the same method as described in Example 8, change in thicknessof the battery was measured after the battery was stored at a hightemperature. The result is shown in FIG. 21. Additionally, theoverdischarge test result of the battery is shown in Table 2 and FIG.22.

COMPARATIVE EXAMPLE 3

A battery was manufactured in the same manner as described in Example 8,except that no additive for a cathode active material was used. By usingthe same method as described in Example 8, change in thickness of thebattery was measured after the battery was stored at a high temperature.The result is shown in FIG. 21. Additionally, the overdischarge testresult of the battery is shown in Table 2 and FIG. 13.

TABLE 2 Discharge Discharge Capacity capacity before capacity afterrestorability after overdischarge overdischarge overdischarge (0.2 C)/mA(0.2 C)/mA % Ex. 12 734 712 97.0% Comp. Ex. 3 728 464 63.7% Ex. 8 729715 98.1% Ex. 9 717 704 98.2% Ex. 10 728 702 96.4% Ex. 11 715 685 95.8%

As shown in FIGS. 15, 17 and 19, each additive for a cathode activematerial for improving overdischarge characteristics, obtained fromExamples 8 and 11, comprises a lithium nickel oxide surface-coated witha material other than lithium nickel oxides. However, as can be seenfrom FIGS. 14, 16 and 18, such surface-coating does not change thestructure of each additive for a cathode active material for improvingoverdischarge characteristics, significantly.

Additionally, as shown in Table 2 and FIGS. 20 and 22, each batteryobtained from Examples 8 to 12 shows similar charge/dischargeefficiency. Capacity of each battery is not significantly reduced evenafter overdischarge and each battery has excellent capacityrestorability after overdischarge. On the other hand, as shown in Table2 and FIG. 13, the battery obtained from Comparative Example 3 (to whichthe additive for a cathode active material for improving overdischargecharacteristics according to the present invention is not added) showspoor overdischarge characteristics compared to other batteries.

However, as can be seen from FIG. 21, Example 8 shows little change inbattery thickness. Examples 9 to 11 show smaller change in batterythickness compared to Example 12. This indicates that each additive fora cathode active material for improving overdischarge characteristicsaccording to Examples 8 to 11 provides excellent effect of preventing abattery from swelling at a high temperature compared to the additiveaccording to Example 12.

While this invention has been described in connection with what ispresently considered to be the most practical and preferred embodiment,it is to be understood that the invention is not limited to thedisclosed embodiment and the drawings. On the contrary, it is intendedto cover various modifications and variations within the spirit andscope of the appended claims.

What is claimed is:
 1. A lithium secondary battery comprising a cathode,an anode, a separator and a non-aqueous electrolyte containing a lithiumsalt and an electrolyte compound, wherein the cathode comprises thecathode active material containing a lithium transition metal oxidecapable of lithium ion intercalation/deintercalation, which furthercomprises a lithium nickel oxide represented by Li₂Ni_(0.97)Al_(0.03)O₂or Li₂Ni_(0.97)B_(0.03)O₂, as an additive in an amount of 0.1 to 5 partsby weight based on 100 parts by weight of the total cathode activematerial, wherein the lithium transition metal oxide is at least oneselected from the group consisting of LiCoO₂, LiNiO₂, LiMnO₂, LiMn₂O₄,Li(Ni_(a)Co_(b)Mn_(c))O₂, LiNi_(1−d)Co_(d)O₂, LiCo_(1−d)Mn_(d)O₂,LiNi_(1−d)Mn_(d)O₂, Li(Ni_(x)Co_(y)Mn_(z))O₄, LiMn_(2−n)Ni_(n)O₄,LiMn_(2−n)Co_(n)O₄, LiCoPO₄ and LiFePO₄, wherein 0<a<1, 0<b<1, 0<c<1,a+b+c=1, 0≦d<1, 0<x<2, 0<y<2, 0<z<2, x+y+z=2, and 0<n<2, and wherein acapacity restorability after overdischarge of the lithium secondarybattery is more than 90% and less than or equal to 94.2%.
 2. The lithiumsecondary battery according to claim 1, wherein the lithium nickel oxidebelongs to a space group Immm.
 3. The lithium secondary batteryaccording to claim 2 wherein the lithium nickel oxide forms atetra-coordinated planar structure, and two tetra-coordinated planarstructures facing to each other share one side formed by O—O so as toform a primary chain.
 4. The lithium secondary battery according toclaim 2, wherein the lithium nickel oxide has the following latticeconstants: a=3.7±0.5 Å, b=2.8±0.5 Å, c=9.2±0.5 Å, a=90°, β=90° and γ=90.5. The lithium secondary battery according to claim 1, wherein thelithium salt is at least one selected from the group consisting ofLiClO₄, LiCF₃SO₃, LiPF₆, LiBF₄, LiAsF₆ and LiN(CF₃SO₂)₂, and theelectrolyte compound is at least one carbonate selected from the groupconsisting of ethylene carbonate, propylene carbonate,gamma-butyrolactone, diethyl carbonate, dimethyl carbonate, ethylmethylcarbonate and methylpropyl carbonate.
 6. The lithium secondary batteryaccording to claim 1, wherein the additive is present in an amount of0.1 to 2 parts by weight based on 100 parts by weight of the totalcathode active material.
 7. The lithium secondary according to claim 1,wherein the capacity restorability after overdischarge of the lithiumsecondary battery is more than 92.3%.
 8. The lithium secondary batteryaccording to claim 1, wherein the capacity restorability afteroverdischarge of the lithium secondary battery is more than 92.3% andless than or equal to 94.2%.
 9. The lithium secondary battery accordingto claim 1, wherein the lithium transition metal oxide is LiCoO₂.